Viewing glasses of this general type can be put in place on connecting branches of vacuum apparatus that are equipped with a flange in order to allow visual inspection of certain sections of the vacuum apparatus.
In one such viewing glass, a glass pane is glued to one of the two face ends of a tube section. The other side of the tube section includes a flange that serves to fasten the viewing glass to a flange of the connecting branch of the vacuum apparatus, so as to maintain a vacuum-tight seal. The manufacture of the vacuum-type glass/metal glued connection is complicated. Further, there is the disadvantage that the distance between the glass pane and the inside of the receiving portion of the vacuum apparatus is relatively large as a consequence of the tube section between pane and flange, so that the viewing angle is restricted.